Chemical-mechanical planarization ("CMP") processes remove materials from the surface layer of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer presses against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution has abrasive particles that abrade the surface of the wafer, and chemicals that oxidize and/or etch the surface of the wafer. Thus, when relative motion is imparted between the wafer and the pad, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals in the slurry (chemical removal).
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer because it is important to accurately focus optical or electromagnetic circuit patterns on the surface of the wafer. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-pattern to within a tolerance of approximately 0.5 .mu.m. Focusing the photo-patterns to such small tolerances, however, is very difficult when the distance between the emission source and the surface of the wafer varies because the surface of the wafer is not uniformly planar. In fact, several devices may be defective on a wafer with a non-uniform planar surface. Thus, CMP processes must create a highly uniform, planar surface.
In the competitive semiconductor industry, it is also desirable to maximize the throughput of the finished wafers and minimize the number of defective or impaired devices on each wafer. The throughput of CMP processes is a function of several factors, one of which is the rate at which the thickness of the wafer decreases as it is being planarized (the "polishing rate") without sacrificing the uniformity of the planarity of the surface of the wafer. Accordingly, it is desirable to maximize the polishing rate within controlled limits.
The polishing rate of CMP processes may be increased by increasing the proportion of abrasive particles in the slurry, solution. Yet, one problem with increasing the proportion of abrasive particles in colloidal slurry solutions is that the abrasive particles tend to flocculate when they are mixed with some desirable oxidizing and etching chemicals. Although stabilizing chemicals may prevent flocculation of the abrasive particles, the stabilizing chemicals are generally incompatible with the oxidizing and etching chemicals. Thus, it is desirable to limit the proportion of abrasive particles in the slurry, solution.
One desirable solution for limiting the proportion of abrasive particles in the slurry is to suspend the abrasive particles in the pad. Conventional suspended particle pads are made by admixing the abrasive particles into a matrix material made from monomer chains. An ionic adhesion catalyst, such as hexamethyldisalizane, may be used to enhance adhesion between the particles and the monomer chains. After the abrasive particles are mixed into the matrix material, the matrix material is cured to harden the pad and suspend the abrasive particles throughout the matrix material. In operation, the suspended abrasive particles in the pad abrade the surface of the wafer to mechanically remove material from the wafer.
One problem with conventional suspended particle polishing pads is that the abrasiveness of the planarizing surface of the pad, and thus the polishing rate of a wafer, varies from one area to another across the surface of the pad. Before the matrix material is cured, the abrasive particles commonly agglomerate into high density clusters, causing a non-uniform distribution of abrasive particles throughout the pad. Therefore, it would be desirable to develop a suspended particle polishing pad with a uniform distribution of abrasive particles throughout the pad.
Another problem with conventional suspended particle polishing pads is that they tend to scratch the surface of the wafer. As the pad planarizes a wafer, the matrix material adjacent to abrasive particles on the planarizing surface of the polishing pad wears down; eventually, some of the abrasive particles break away from the pad and travel in the slurry. Particles also break away from pads with ionic adhesion catalysts because electrostatic solvents weaken the ionic bonds between the matrix material and the particles. When a large agglomeration of suspended particles breaks away from the pad, it may scratch the surface of the wafer and seriously damage several of the devices on the wafer. Therefore, it would be desirable to develop a pad that substantially prevents abrasive particles from breaking away from the pad.